1. Field of the Invention:
The present invention relates generally to a device for locking a tripod having three identical legs each composed of a fixed leg and a movable leg coupled together in a telescopic manner, and more particularly to such a tripod lock device which is capable of locking each leg of the tripod by one motion and is particularly suitable for use with a tripod for surveying instruments.
2. Description of the Prior Art:
Conventionally, a tripod used with a surveying instrument includes a head and three legs hinged at one end to the head. Each of the legs includes a fixed leg composed of two parallel spaced fixed leg members, and a movable leg slidably disposed between the fixed leg members for achieving a telescopic motion. The movable leg can be locked in position against displacement relative to the fixed leg by means of a lock member such as a screw fastener associated with one leg member of the fixed leg.
When the three legs of the tripod are to be bound together, a fastening strip or band is used in addition to the lock member. The band has a buckle at one end and a punch hole at an opposite end. The punch hole is engageable with a prong of the buckle. In use, the band is fastened around the legs of the tripod and then the prong of the buckle is engaged into the punch hole, thereby binding the legs together against spreading.
The screw fastener used as the lock member between the fixed leg and the movable leg may include a screw having a diameter 4 mm and a pitch of 0.7 mm, and a screw having a diameter of 8 mm and a pitch of 1.25 mm. In order to provide an effective fastening strength, the screw must be advanced by a distance at least equal to one pitch of the screw. This means that the screw must be rotated in a tightening direction at least by one turn. The tightening operation of the screw fastener is tedious and time-consuming. In addition, the lock member composed of a screw fastener having threads is likely to catch a foreign matter such as dirt which will stain the screw fastener and thereby lower the durability of the screw fastener. Due to the dirt deposited between the adjacent threads, the screw fastener cannot rotate smoothly and only gives an insufficient tightening strength.
The conventional lock member is able to provide a firm lock between the fixed leg and the movable leg of each of the three legs, however, in order to lock the three legs together, a separate fastener member such as a fastening band must be provided. This means that in the case of the conventional tripod, fastening between the fixed leg and the movable leg of each of the three legs and fastening between the three legs are accomplished separately. In the case where a fastening band is used, each time when hand-carrying of a tripod from one survey point to another or transportation of the tripod for a long distance is necessary, the fixed leg and the movable leg of each leg are fastened together by one lock member and, subsequently, the three legs are bundled together by the fastening band. In this instance, one end of the fastening band is threaded through the buckle and, subsequently, the prong of the buckle is fitted into the punch hole in the fastening band. The legbinding operation using the fastening band makes the hand-carrying of transportation of the tripod tedious and inconvenient.